For many years it has been an aim of scientists in the field of specifically targeted drug therapy to use monoclonal antibodies (MAbs) for the specific delivery of toxic agents to human cancers. Conjugates of tumor-associated MAbs and suitable toxic agents have been developed, but have had mixed success in the therapy of cancer, and virtually no application in other diseases, such as infectious and autoimmune diseases. The toxic agent is most commonly a chemotherapeutic drug, although particle-emitting radionuclides, or bacterial or plant toxins have also been conjugated to MAbs, especially for the therapy of cancer (Sharkey and Goldenberg, CA Cancer J Clin. 2006 July-August; 56(4):226-243) and, more recently, with radioimmunoconjugates for the preclinical therapy of certain infectious diseases (Dadachova and Casadevall, Q J Nucl Med Mol Imaging 2006; 50(3):193-204).
The advantages of using MAb-chemotherapeutic drug conjugates are that (a) the chemotherapeutic drug itself is structurally well defined; (b) the chemotherapeutic drug is linked to the MAb protein using very well defined conjugation chemistries, often at specific sites remote from the MAbs antigen binding regions; (c) MAb-chemotherapeutic drug conjugates can be made more reproducibly than chemical conjugates involving MAbs and bacterial or plant toxins, and as such are more amenable to commercial development and regulatory approval; and (d) the MAb-chemotherapeutic drug conjugates are orders of magnitude less toxic systemically than radionuclide MAb conjugates.
Early work on protein-drug conjugates indicated that a drug preferably is released in its original form, once it has been internalized into a target cell, for the protein-drug conjugate to be a useful therapeutic. Trouet et al. (Proc. Natl. Acad. Sci. USA 79:626-629 (1982)) showed the advantage of using specific peptide linkers, between the drug and the antibody moiety, which are cleaved lysosomally to liberate the intact drug. Notably, MAb-chemotherapeutic drug conjugates prepared using mild acid-cleavable linkers, such as those containing a hydrazone, were developed, based on the observation that the pH inside tumors was often lower than normal physiological pH (Willner et al., U.S. Pat. No. 5,708,146; Trail et al. (Science 261:212-215 (1993)). The first approved MAb-drug conjugate, gemtuzumab ozogamicin, incorporated an acid-labile hydrazone bond between an anti-CD33 antibody, humanized P67.6, and a potent calicheamicin derivative. Sievers et al., J Clin Oncol. 19:3244-3254 (2001); Hamann et al., Bioconjugate Chem. 13: 47-58 (2002). In some cases, the MAb-chemotherapeutic drug conjugates were made with reductively labile hindered disulfide bonds between the chemotherapeutic drugs and the MAb (Liu et al., Proc Natl Acad Sci USA 93: 8618-8623 (1996)).
Yet another cleavable linker involves cathepsin B-labile dipeptide spacers, such as Phe-Lys or Val-Cit, similar to the lysosomally labile peptide spacers of Trouet et al. containing from one to four amino acids, which additionally incorporated a collapsible spacer between the drug and the dipeptide (Dubowchik, et al., Bioconjugate Chem. 13:855-869 (2002); Firestone et al., U.S. Pat. No. 6,214,345 B1; Doronina et al., Nat Biotechnol. 21: 778-784 (2003)). The latter approaches were also utilized in the preparation of an immunoconjugate of camptothecin (Walker et al., Bioorg Med Chem Lett. 12:217-219 (2002)). Another cleavable moiety that has been explored is an ester linkage incorporated into the linker between the antibody and the chemotherapeutic drug. Gillimard and Saragovi have found that when an ester of paclitaxel was conjugated to anti-rat p75 MAb, MC192, or anti-human TrkA MAb, 5C3, the conjugate was found to exhibit target-specific toxicity. Gillimard and Saragovi, Cancer Res. 61:694-699 (2001).
Current notions of antibody-drug conjugate design emphasize the use of ultratoxic drugs attached to antibodies using stable bonds that are cleaved only intracellularly. This approach has been used to design conjugates of ultratoxic drugs, such as calicheamicin, monomethylauristatin-E (MMAE), and maytansinoids. Although very stable bonding to MAbs results in stability in circulation, the conjugates are also processed in liver, spleen, and kidney, thereby releasing the toxic drugs in those organs and potentially reducing the therapeutic window in disease treatment applications. While recent regulatory approval of ADCETRIS® (brentuximab vedotin) for Hodgkin's lymphoma and of KADCYLA® (ado-trastuzumab emtansine) for refractory breast cancer are encouraging, the lack of therapeutic efficacy in the maximum administrable dosage of calicheamicin conjugate in non-Hodgkin lymphoma, and its subsequent discontinuation, as well as the market withdrawal of gemtuzumab ozogamicin for AML point to the limitations of using ultratoxics in ADCs.
The conjugates of the instant invention possess greater efficacy than unconjugated or “naked” antibodies or antibody fragments, although such unconjugated antibody moieties have been of use in specific situations. In cancer, for example, naked antibodies have come to play a role in the treatment of lymphomas (alemtuzumab and rituximab), colorectal and other cancers (cetuximab and bevacizumab), breast cancer (trastuzumab), as well as a large number now in clinical development (e.g., epratuzumab, veltuzumab, milatuzumab). In most of these cases, clinical use has involved combining these naked, or unconjugated, antibodies with other therapies, such as chemotherapy or radiation therapy.
Use of CL2A linkers to attach therapeutic drugs, such as SN-38, to antibody moieties has been disclosed (e.g. U.S. Pat. Nos. 7,999,083 and 8,080,250, the Examples sections incorporated herein by reference). However, a need exists for more efficient methods of preparing and using CL2A and MAb-CL2A-SN-38 conjugates, including optimized dosage schedules that result in maximal efficacy and minimal toxicity, as well as efficient large scale production of CL2A-SN-38 and antibody-CL2A-SN-38 conjugates.